Host control of insect endogenous retroviruses: small RNA silencing and immune response

Endogenous retroviruses are relics of ancient infections from retroviruses that managed to integrate into the genome of germline cells and remained vertically transmitted from parent to progeny. Subsequent to the endogenization process, these sequences can move and multiply in the host genome, which can have deleterious consequences and disturb genomic stability. Natural selection favored the establishment of silencing pathways that protect host genomes from the activity of endogenous retroviruses. RNA silencing mechanisms are involved, which utilize piRNAs. The response to exogenous viral infections uses siRNAs, a class of small RNAs that are generated via a distinct biogenesis pathway from piRNAs. However, interplay between both pathways has been identified, and interactions with anti-bacterial and anti-fungal immune responses are also suspected. This review focuses on Diptera (Arthropods) and intends to compile pieces of evidence showing that the RNA silencing pathway of endogenous retrovirus regulation is not independent from immunity and the response to infections. This review will consider the mechanisms that allow the lasting coexistence of viral sequences and host genomes from an evolutionary perspective.